Why 12% of Our Signups Were Fake — and What We Did About It

Why 12% of Our Signups Were Fake — and What We Did About It Last October, I opened our ESP dashboard and saw 12.3% hard bounces on onboarding emails. Not soft bounces. Not mailbox full. Hard 550 rejections. The kind that make your provider send you a warning email that feels like a threat. We weren't spamming anyone. We weren't scraping lists. These were users who had just signed up. That's when it clicked. This wasn't a sending problem. It was a signup problem. What's Actually Happening When an Email Bounces Most developers don't look at the SMTP layer until something breaks. I didn't either. When your server sends mail, it connects to the recipient domain's MX record. The flow usually looks like this: 1. TCP connect 2. EHLO your-domain.com 3. MAIL FROM:<you@your-domain.com> 4. RCPT TO:<user@example.com> If the mailbox doesn't exist, the remote server replies with something like: 550 5.1.1 User unknown That rejection often happens before any message body is sent. No content. No HTML.